For precise dosing of extremely high-quality media, for example in the pharmaceuticals sector, the complex and expensive dosing units must be cleaned and sterilized with great effort. This is worthwhile primarily in the case of large sample quantities.
In such dosing units, pump fluid, especially air, is alternatingly blown in the pump chamber and drawn out therefrom, such that the diaphragm is moved between two end positions and the volume of the dosing chamber facing the pump chamber thus increases and decreases. This results in a diaphragm pump via which the dosing fluid flows out of or is drawn into the dosing unit in very precise quantities. The dosing chamber is fluidically connected or disconnected, on the feed side and on the drain side, with the feed and the drain, respectively, by at least one diaphragm valve each. The diaphragm valves are controlled, for example, by a pneumatic valve that controls the application of pressure to the diaphragm on the side facing the dosing fluid.
From DE 10 2008 028 772 A1, a pneumatic dosing unit is known in which the quantity of a dosing fluid is controlled by the opening time of a diaphragm valve. A second diaphragm valve is provided downstream of the diaphragm valve, which second diaphragm valve, however, can be circumvented by a bypass channel of a defined size. When the second diaphragm valve is closed, the dosing occurs in that fluid flows through the precisely dimensioned bypass channel. The second diaphragm valve serves to suck the line between the valves empty for a short time after the first diaphragm valve is closed. For a precise dosing of the fluid, it is necessary to know the flow speed of the fluid.